A vehicle, particularly a motor vehicle, using assist torque for lane-maintaining during driving of a vehicle is increasing. Such assist torque is given for performing steering toward a direction (i.e., a direction toward a lane center position) opposite to a direction of a lane deviation so that the lane-maintaining is supported, i.e., the lane deviation is prevented. A steering force toward a direction in which the assist torque for lane-maintaining is applied is reduced, and thus a driver is naturally made return to the lane center position. Of course, the magnitude of the assist torque for lane-maintaining is limited within a range of magnitude that the driver can overcome without being prevented from performing steering for actively changing the lane.
In such lane-maintaining control, typically, a camera or similar is used to detect a lane width of a traveling lane on which the host vehicle is traveling. Then, a guidance target point is set so that the host vehicle travels along a traveling line formed by connecting middle positions of the lane widths detected.
During the lane-maintaining control, the front of the driving lane on which the host vehicle is traveling might be branched (e.g., bifurcated). In this case, the lane width detected by the lane width detection unit gradually increases at the branching portion. It is suggested to interrupt the lane-maintaining control when the lane width gradually increases in that manner, because the vehicle is on the branch part and it is unclear which road the driver wants to travel unless a turn signal is operated (See Patent Document 1).